Method for solubilizing silica

Abstract: A method for solubilizing colloidal silica in aqueous systems containing silica is provided wherein a silica-containing solution is diluted, one or more chelants are added and the pH is adjusted to be above 10.

Description: Silica is one of the major fouling problems in many processes using water and it can assume many low solubility chemical forms depending on the conditions. Below about pH 7 for example, monomeric silica tends to polymerize to form oligomeric or colloidal silica. At higher pHs particularly above about pH 9.5, silica can form monomeric silicate ion. Since conversion can be slow, all of these forms may exist at any one time depending on the history of the system. Furthermore, the silicate ion can react with polyvalent cations like magnesium and calcium commonly present in process waters to produce salts with very limited solubility. Thus it is common for a mixture of many forms to be present: monomeric, oligomeric and colloidal silica; magnesium silicate, calcium silicate and other silicate salts. In describing this complex system, it is common practice to refer to the mixture merely as silica or as silica and silicate. Herein these terms are used interchangeably. In order to prevent the problems associated with the formation of silica scale, industrial engineers generally employ one of three general approaches: limit the amount of silica present in the system, use scale prevention agents, or a combination thereof. Limiting the level of silica introduced or allowed to accumulate in the aqueous system is still the primary method of dealing with the problem. When utilizing any of these three approaches, it is desirable to accurately know the level of silica present in the system. When relying on limiting the level of silica present in the system, if the system is flushed or replenished with make-up water when the silica levels are still low, energy and time are wasted. If the system is flushed or replenished with make-up water after the silica levels have risen too high, scale may already have formed. Similarly, it is desirable to know the level of silica so that scale prevention agents can be used economically. It would de desirable to be able to accurately measure the level of silica in an aqueous systems such as, for example: cooling water, boiler water, geothermal processing (used to generate electricity or for heating), and sugar (particularly cane and beet) processing. In each of these processes, heat is transferred to or from the water. In three of these systems, cooling water, boiler water and sugar processing, heat is added to the water and evaporation of some of the water takes place. As the water is evaporated the silica (or silicates) will concentrate. If the silica concentration exceeds its solubility, it can deposit to form either a vitreous coating or an adherent scale that can normally be removed only by laborious mechanical or chemical cleaning. In geothermal processes, hot water laden with silica or silicates is used to heat homes or factories or is converted to steam to drive a turbine and generate electricity. At some point in each of the above four processes, heat is extracted from the water, making any dissolved silicate less soluble and thus likely to deposit on surfaces. The current practice in these processes is to mechanically limit the amount of silica or silicates that build up in the water so that the catastrophic consequences of deposition of these compounds does not occur. For example, in cooling water, the accepted practice is to limit the amount of silica or silicates to about 180 ppm, expressed as SiO 2 . In addition, deposition of CaCO 3 (which can act as a nucleating agent for silica or silicates to deposit upon) is controlled by well known inhibitors such as phosphonates or polymers such as polyacrylic acid or polymaleic acid. Because the silica is limited to 180 ppm and because in many arid areas of the US and other parts of the world make-up water may contain from 50-100 ppm silica, cooling water can only be concentrated 2 to 3 times before the risk of silica or silicate deposition becomes too great. Similarly in boiler water, the American Society of Mechanical Engineers recommends that silica be limited to certain levels depending on the operating pressure of the boiler. For example, in low pressure boilers (less than 300 psig) the amount of silica, as SiO 2 , should be kept below 150 ppm. As the pressure is raised, the level of silica that can be tolerated in the recirculating boiler water becomes progressively less. A polymer that would enable boilers to operate at higher cycles of concentration, particularly low pressure boilers where silica volatilization is not a great concern, would allow more energy-efficient use of the heated water. Thus it is desirable to be able to determine the level of silica in these systems to make efficient use of the method of control, whether by limiting the amount of silica present or by adding scale control agents, and to minimize the risk of exceeding the recommended limits of silica levels. Chemerys, "Comparison of Analytical Methods for the Determination of Silica in Geothermal Water," Journal of Volcanology and Geothermal Research, Vol. 16, (1983) pages 57-63 discloses that for acurate colorimetric results, geothermal samples containing silica must be diluted in the field to ensure that no further polymerization occurs and that amorphous silica that may be present will redissolve. Chemerys further states that if the samples are not diluted in the field, they should be diluted in the laboratory and left standing for at least a month to allow the silica to redissolve; if analyzed immediately, the diluted samples should be made alkaline and heated overnight in a 90°C oven. Chemerys also states that shaking seems to give better precision and accuracy, and that if the samples are not properly diluted, heating them with alkaline treatment prior to analysis gives more accurate results. In SU 1190257A, a gravimetric method is described for the selective determination of calcium silicate in the presence of silica in building materials. As part of that method, a sample the building material to be analysed is triturated in a mortar in the presence of ethylenediamine tetraacetic acid (Complexone). After that, the contents of the mortar are refluxed for 10 to 60 minutes, cooled, diluted with water, and filtered to yield a solid precipitate. The solid precipitate is then washed and calcined to constant weight. The disclosure has little to do with the field of the present invention, i.e. the quantitative determination of total water soluble silica in an aqueous sample containing one or more of water soluble silica, colloidal silica and water soluble silicate. Known analytical methods for determining the level of silica in a sample include colorimetric methods, spectrophotometric methods, inverse phase chromatographic methods, atomic absorption methods, direct current plasma analysis and inductively coupled plasma amalysis. As practiced at the moment, however, these methods suffer from one or more drawbacks since they are inaccurate by not taking into account the various forms in which the silica may be present in the sample. It is known to fully solubilize any colloidal silica, which may be present in a sample, by treatment of the sample with fluoride ions, such as with hydrofluoric acid; however this is an extremely hazardous and time consuming method. The object of the invention is therefore to provide an improved method for the quantitative determination of total silica in a sample, that is to say the total content of water soluble silica, colloidal silica, and water soluble silicate. In broad terms, therefore, the present invention provides a method for the quantitative determination of total soluble silica, that is to say water soluble silica, water soluble silicate and colloidal silica, present in solution in an aqueous sample containing one or more thereof, by colorimetric analysis, spectrophotometric analysis, inverse phase chromatography, atomic absorption analysis, direct current plasma analysis or inductively coupled plasma amalysis, wherein the sample is treated by the preliminary steps of: i) diluting the sample to bring the anticipated soluble silica content of the original sample down to a level detectable by the particular measurement technique to be used, and referred to hereinafter as an "expected" detectable level; ii) adding to the sample, diluted as in step i), a chelating agent capable of binding metal atoms in the said sample; iii) if necessary, adding to the sample containing the said chelating agent a water-soluble inorganic or inorganic base in an amount sufficient to raise the pH of the sample to a value in excess of 10; and iv) heating and/or agitating the pH adjusted, chelating agent-containing sample thereby to solublize any colloidal silica present in the said sample prior to the said silica determination. As will be apparent to the person skilled in the art, for each measurement technique, there is an associated range of silica levels for which the technique can provide reliable, quantitative measurements. As used herein, "detectable silica level" refers to such a range; in other words, the range of silica levels for which the analytical technique can provide reliable, quantitative measurements of the level of silica. For example, the Hach Silicomolybdate Method, a spectrophotometric technique, can reliably measure soluble silica levels up to about 30 parts per million (ppm). Often the level of silica in a sample is unknown. However, those skilled in the art can approximate the level of silica in a system based on factors such as the geographical location of the water supply, the number of times the water has been recycled and other factors. From that approximation, the degree of dilution required to achieve an "expected detectable level" can easily be determined. In addition to diluting the silica-containing sample to an expected detectable level, the method of the present invention utilizes one or more chelating agents. Such agents are well known to those skilled in the art of water treatment and are materials which bind metal atoms in solution. Suitable chelating agents, also referred to hereinafter as chelants, include any material with a sufficiently high stability constant, or binding affinity, to prevent cations, for example, magnesium or iron cations from forming as their hydroxide, oxide or other anion salts at the elevated pH of the system. Examples of suitable chelants include ethylenediamine tetraacetic acid, nitrilotriacetic acid, N-(hydroxyethyl)-ethylenediamine triacetic acid, aminotris(methylenephosphonic) acid, 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, diethyene triamine pentamethylene phosphonate, diethylenetriamine pentaacetic acid, triethanolamine, ethylenediamine tetra(methylenephosphonic) acid, hexamethylenediamine tetra(methylenephosphonic) acid, and the partial and complete alkali metal and ammonium salts thereof. Preferred chelants are, for example, ethylenediamine tetraacetic acid, nitrilotriacetic acid, N-(hydroxyethyl)-ethylenediamine triacetic acid and the partial and complete sodium salts thereof. The one or more more chelants are preferably added to the silica-containing sample at a level of from about 10 to about 50,000 ppm, and most preferably from about 100 to about 5,000 ppm. The preferred level of chelant in any given sample may depend upon the level of chelatable metal species present in the sample; as the level of chelatable metal species increases, the preferred level of chelant increase. The method of the present invention requires that the pH of the sample be above 10. This can be accomplished, for example, by the addition of any water-soluble organic or inorganic base. When a base is added to the sample to adjust the pH, it is preferred that the base is an alkali metal or ammonium salt, such as sodium hydroxide, potassium hydroxide or ammonium hydroxide. Most preferably, when a base is added to the sample to adjust the pH, it is sodium hydroxide. The pH of the sample may be above 10 as a result of the addition of the chelant. For example, the addition of the chelating agents such as tetrasodium salt of ethylenediamine tetraacetic acid or the tetrasodium salt of hydroxyethylenediamine tetraacetic acid will generally raise the pH of the sample. If the pH of the sample is above 10, the addition of a water-soluble organic or inorganic base is optional. In one embodiment of the present invention, the silica-containing sample at a pH above 10 is heated after adding the one or more chelants. The sample can be heated by any conventional means such as for example, placing the container with the silica-containing sample in an oven, water-bath, or oil oil-bath. It is desirable to ensure that any loss of water due to evaporation is minimized to prevent the silica from becoming concentrated, thus resulting in an artificially high silica level measurement. The silica-containing sample is preferably heated to a temperature of from about 30°C to about 100°C, and most preferably from about 40°C to about 70°C. In another embodiment of the present invention, the silica-containing sample at a pH above 10 is agitated after adding the one or more chelants. The sample can be agitated by any conventional means such as for example, placing the container with the silica-containing sample on a shaker or roller. Preferably, the silica-containing sample at a pH above 10 is agitated and heated after adding the one or more chelants. Preferably, the silica-containing sample at a pH above 10 is maintained in the presence of the one or more chelants for a period of time before the level of silica is measured. Preferably, the silica-containing sample at a pH above 10 is maintained in the presence of the one or more chelants for a period of from about 30 minutes to about 36 hours, and most preferably from about 45 minutes to about 24 hours before the level of silica is measured. As a result of the present invention, silica is solubilized reasonably quickly, easily and inexpensively. Thus, quantitative determinations of the level of silica in a silica-containing sample can be made accurately and quickly by any available analytical method including colorimetric methods, spectrophotometric methods, inverse phase chromatographic methods and atomic absorption methods. Preferably, the analytical method is a colorimetric or spectrophotometric method. The following test was employed to demonstrate the method of the present invention for solubilizing silica. The test procedure used was one which simulated silica-laden process water conditions. An aqueous silica stock solution was prepared in the following manner: 3.48 grams N® Clear Sodium Silicate (a trademark of the PQ Corporation) (28.7 percent by weight aqueous SiO 2 ) was diluted to 1 litre with deionized water. An aqueous sodium bicarbonate stock solution was prepared in the following manner: 5.60 grams of NaHCO 3 was diluted to 1 litre with deionized water. An aqueous calcium/magnesium stock solution was prepared in the following manner: 1.83 grams CaCl 2 ·2H 2 O and 1.52 grams of MgCl 2 ·6H 2 O were diluted to 1 litre with deionized water. Simulated cooling water was prepared in the following manner: To a 4 ounce plastic jar was added : 40 grams of the silica stock solution 15 grams of the sodium bicarbonate stock solution 40 grams of the calcium/magnesium stock solution 5 grams of deionized water The simulated cooling water was allowed to stand at room temperature for 11 days. The levels of the various components of the simulated cooling water were: 400 ppm Si as SiO 2 300 ppm Mg 2+ (as CaCO 3 ) 500 ppm Ca 2+ (as CaCO 3 ) After standing at room temperature for 11 days, a 30 gram sample of the simulated cooling water was taken and diluted to 1,000 grams with deionized water. Example 1 The silica level of the diluted simulated cooling water was measured by the Hach Silicomolybdate Method which involves the reaction of molybdate ion with silica and phosphate to form a yellow color. Citric acid is added to destroy the phosphomolybdic acid complex but not the silicomolybdic acid complex. The level of silicomolybdate acid complex is determined by spectrophotometric techniques (total absorbance at 410.0 nm). The data for Example 1 appearing in Table I, below, is the measured value (Hach method) of the level of silica taking into account the dilution of the sample. The measurement was repeated after 1 hour, 4 hours and 24 hours. Example 2 was conducted in a similar manner as Example 1 except that to a 100 gram sample of the diluted simulated cooling water was added 0.4 grams of a 1 percent by weight aqueous sodium hydroxide solution. Example 3 was conducted as Example 1 except that to a 100 gram sample of the diluted simulated cooling water was added 1.0 gram of a 40 percent by weight aqueous solution of ethylenediaminetetraacetic acid, tetrasodium salt. Example 4 was conducted as Example 1 except that to a 100 gram sample of the diluted simulated cooling water were added 0.4 grams of a 1 percent by weight aqueous sodium hydroxide solution and 1.0 gram of a 40 percent by weight aqueous solution of ethylenediaminetetraacetic acid, tetrasodium salt. Example 5 was conducted as Example 1 except that to a 100 gram sample of the diluted simulated cooling water were added 0.2 grams of a 1 percent by weight aqueous sodium hydroxide solution and 0.5 grams of a 40 percent by weight aqueous solution of ethylenediaminetetraacetic acid, tetrasodium salt. Example 6 was conducted as Example 4 except that the sample was placed on a mechanical shaker and was continuously shaken. Example 7 was conducted as Example 4 except that the sample was maintained at 50°C. Example 8 was conducted in a similar manner as Example 4 except that the sample was placed on a mechanical roller and was continuously rolled while maintaining the sample at 50°C. MEASURED SILICA LEVELS IN SAMPLES CONTAINING 400 ppm SILICA Measured SiO 2 Level (ppm) Ex Conditions 0 hr 1 hr 4 hrs 24 hrs pH 1 Dilution only 137 143 173 370 9.3 2 Dilution and 0.004% NaOH 150 167 403 11.0 3 Dilution, 0.4% Na 4 EDTA 216 360 403 10.9 4 Dilution, 0.4% Na 4 EDTA and 0.004% NaOH 186 367 400 11.2 5 Dilution, 0.2% Na 4 EDTA and 0.002% NaOH 206 400 410 11.0 6 Dilution, 0.4% Na 4 EDTA, 0.004% NaOH and shaking 196 393 397 11.2 7 Dilution, 0.4% Na 4 EDTA, 0.004% NaOH, 50°C 383 443 423 11.2 8 Dilution, 0.4% Na 4 EDTA, 0.004% NaOH, rolling, 50°C 406 417 470 11.2 As can be seen by the data in Table I, above, the silica in the silica-containing samples at pH 10 or above are substantially solubilized in the presence of a chelant in a reasonable period of time.

Claims: A method for the quantitative determination of total soluble silica, consisting of water soluble silica, water soluble silicate and colloidal silica, present in solution in an aqueous sample containing one or more thereof, by colorimetric analysis, spectrophotometric analysis, inverse phase chromatography, atomic absorption analysis, direct current plasma analysis or inductively coupled plasma amalysis, comprising the preliminary steps of: i) diluting the sample to bring the anticipated soluble silica content of the original sample down to a level detectable by the particular measurement technique to be used; ii) adding to the sample, diluted as in step i), a chelating agent capable of binding metal atoms in the said sample; iii) if necessary, adding to the sample containing the said chelating agent a water-soluble inorganic or inorganic base in an amount sufficient to raise the pH of the sample to a value in excess of 10: and iv) heating and/or agitating the pH adjusted, chelating agent-containing sample thereby to solublize any colloidal silica present in the said sample prior to the said silica determination. A method according to claim 1, wherein the chelating agent is ethylenediamine tetraacetic acid, nitriloacetic acid N-(hydroxyethyl)-ethylenediamine triacetic acid, aminotris(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, diethylenetriamine pentamethylene phosphonate, diethylenetriamine pentaacetic acid, triethanolamine, ethylenediamine tetra(methylenephosphonic) acid, or hexamethylenediamine tetra(methylenephosphonic) acid, or a partial or complete alkali metal or ammonium salt thereof. A method according to claim 1. wherein the chelating agent is ethylenediamine tetraacetic acid, nitriloacetic acid or N-(hydroxyethyl)-ethylenediamine tetraacetic acid, or a sodium salt thereof. A method according to any one of the preceding claims, wherein the chelating agent is added to the sample, or the diluted sample, as the case may be, in an amount of from 10 to 50,000 ppm, preferably from 100 to 5000 ppm. A method according to any one of the preceding claims, wherein, in step iv), the sample containing the chelating agent is heated, preferably with agitation, to a temperature in the range of from 30° to 100°C, preferably from 40° to 70°C, for a period of from 30 minutes to 36 hours, preferably from 45 minutes to 24 hours, prior to the determination of the silica content. A method according to any one of the preceding claims, wherein in step iii), the pH of the sample containing the chelating agent is adjusted to a value in excess of 10 by the addition of aqueous sodium hydroxide. A method according to any one of the preceding claims, wherein, following the pretreatment, the silica content of the sample is determined by a colorimetric or spectrophotometric analysis.

Inventors: BARDSLEY JUDY HUHES, HANN WILLIAM MATHIS, BARDSLEY, JUDY HUHES, HANN, WILLIAM MATHIS
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1994
Main classification: G01N1
IPC codes: C02F5, C02F1, G01N1, G01N33